Ceramic body



Patented July l5, 1947 CERAMIC BODY Chester R. Austin, Columbus, Ohio,and Edwin J. Rogers, Troy, N. Y

signments, to United East Hartford, Conn.,

ware, and Ethyl Corporation,

by meine as- Ail-orali Corporation,

a corporation ci Dela- Detroit, Mich.,

a corporation o! Delaware Application November 18, 1944, Serial No.563,664

(Cl. 10B-83) ilclalml.

I'his invention relates to ceramic bodies. More especially. it isconcerned with the production of an electrical insulating material thatis particularly adapted for spark-plug insulators and for uses requiringsimilar qualities.

The increasing use of ceramic insulators in aircraft engines has broughtabout a demand for spark-plug insulators oi' a superior quality.Porcelaln spark-plug insulators were among the iirst used in internalcombustion engines. Later, bodies oi' the muilite type were developed.The present demand for the best possible materials has brought abmit thedevelopment ci' bodies high in alumina. Initially, these bodies weremade of almost pure alumina: however, in such cases ex tremely highburning temperatures of around 3200 F. or higher are required to eiiectthe recrystailiiiation that is necessary to produce the propercharacteristics. Such high temperatures exceed the limits therefore,introduce serious production problems. Mica plugs are unsatisfactory forhigh-output engines.

The ideal spark-plug insulator must be resistant to thermal shock inorder to withstand the marked fluctuations in temperature that occur inservice. It must also have relatively high thermal conductivity so thatsumcient heat is conducted away from the tip end to preventpre-ignition. High thermal expansion is also desirable to insure that itexpands and contracts as uniformly as possible with the metal parts withwhich it is assembled. In addlti oi course, it must maintain itselectrical insulating properties at the elevated temperatures to whichit is subjected in service, and it must resist attack at elevatedtemperatures by lead compounds. However, inasmuch as most spark-plugfailures can be traced, apparently, to insuilicient strength, one of themost important characteristics of the ideal insulator is high mechanicalstrength. Sumcient strength is required to withstand both the stressesproduced during assembly and the stresses that arise during service.

From a manufacturing standpoint, it is desirable that the ceramic bodyof the insulator retain its form and symmetry and that it have a uniformshrinkage. This uniformity makes possible the production of an articlehaving close dimensional tolerances. In addition, the body should matureat a temperature below 3000 F. so that the insulators may be burned incommercially available kilns.

Itis, therefore. one of the objects of our invenmm m nrovide a ceramicbody suitable i'cr sparkof most commercial kilns and, zo

so the attainment of the 2 plug insulators having high mechanicalstrength, high thermal conductivity, high thermal expansion, and goodelectrical insulating properties and resistance to attack by leadcompounds at elevated temperatures.

A further object of our invention is to produce such an insulator by acareful selection of ingredlents to insure that losses occurring throughwarpage during tiring are held to a minimum.

Another object of our invention is to provide, by careful selection ofingredients. an aluminabase ceramic body in which the alumina can bere-crystallised at temperatures below 3000 F.

A further object of our invention is to provide a ceramic body whereingood electrical properties are maintained at elevated temperatures eventhough an appreciable amount of alkalies may be present.

It is well to lower materially known that various fluxes are eliectivethe re-crystallisation temperature of alumina bodies. Such materials asMgO, BaO, CaO, FexOs, and SiO: have been proposed as iluxes for thispurpose; however, none of these fluxes yield an insulator having all ofthe desired characteristics. If suiilclent magnesia is added to lowerthe re-crystallization temperature, n. magnesia spine! crystallizes inthe cubic system and markedly lowers the strength of the resultinginsulator. Additions of barium oxide also prevent desired strength.Calcium oxide, when added in amounts sufficient to decrease the burningtemperature to the desired range, tends to produce bodies having anundesirably short burning range. a rather coarse crystal structure, anda deiinite tendency to warp badly during tiring. The addition ofsignificant amounts of FeaO: tends to result in a rather drastic loss ofelectrical insulating properties at elevated temperatures, particularlyif the body is burned in a reducing atmosphere. Silica. when present inamounts adequate to lower the re-crystallization temperature to thedesired range, tends to cause a marked loss of strength and resistanceto thermal shock.

The above and other objects and advantages of our invention will appearin the following description and appended claims when considered inconjunction with the accompanying drawings which form a part of thisspecification.

In said drawings:

Figure 1 represents a corner of a triaxial diagram showing the relativeproportions of alumina. calcium oxide, and silica used in the ceramicbodies coming within the scope of our invention.

Figure 2 shows a reproduction of a photomicrograph of a typicalthin-section of one of the products of our invention.

Before explaining in detail the present invention it is to be understoodthat the invention is not limited in its application to the detailsillustrated in the accompanying drawings, since the invention 1s capableof other embodiments and of being practised or carried out in variousways. Also it is to be understood that the phraseology or terminologyemployed herein is for the purpose of description and not of limitation,and it is not intended to limit the invention herein clamed beyond therequirements of the prior art.

We have found that alumina bodies having the requisite properties forspark-plug insulators can be produced by the addition of the properamounts of calcium oxide and silica. Although neither oi these materialsis satisfactory when added singly, the desire results are obtained bythe addition of the proper combination of the two as is hereinafterdisclosed.

We have found also that it is particularly advantageous to introduce thecalcium oxide into the body as tri-calcium penta-aluminate. We havefurther discovered that exceptional results are obtained when thetri-calcium pentaaluminate is fused and allowed to crystallize prior toits incorporation in the body.

In Figure 1 of the drawings, the respective sides of the trianglerepresent weight percentages of the three essential components of ourceramic body-alumina, calcium oxide, and silica. The left-hand siderepresents alumina contents varying froxn 80 to 100 per cent: therighthand side represents silica contents varying from to 20 per cent:while the base represents calcium oxide contents varying from 0 to 20per cent.

As mentioned above, we have discovered that particularly desirableproperties may be obtained in ceramic bodies composed essentially ofalumina. calcium oxide, and silica. The compositions giving thesedesirable properties are approximately defined in Figure l by the solidlines AB, BC, CD, DE, and EA, the approximate coordinates oi' the pointsbeing: A-99.l percent alumina, 0.4 per cent calcium oxide, and 0.5 percent silica; B-93.6 per cent alumina, 0.4 per cent calcium oxide, and 6per cent silica: lC-9l.5 per cent alumina, 2.5 per cent calcium oxide,and 6 per cent silica; D-91.5 per cent alumina. 5.5 per cent calciumoxide, and 3 per cent silica; and E-97 per cent alumina, 2.5 per centcalcium oxide, and 0.5 per cent silica. Within the area so defined, anycomposition selected will be suitable for use as spark-plug insulatorsor in other applications requiring similar properties.

Although excellent properties maybe obtained in bodies havingcompositions within the range above described, in order to secure thevery best properties with the optimum handling characteristics incommercial production, we prefer to employ compositions within asomewhat narrower range. We have found that the relative proportions ei'alumina, calcium oxide, and silica in the iinal body may advantageouslybe kept ,within the narrower limits represented in Figure l by the areaapproximately defined by the broken lines FG, GH, HI, and IF, thecoordinates of the points being approximately: F-98.3 per cent alumina,0.7 per cent calcium oxide, and 1 per cent silica: G-94.3 per centalumina,

0.7 per cent calcium oxide, and 5 per cent sllica; H-92.5 per centalumina, 2.5 per cent calcium oxide, and 5 per cent silica; and I96.5per cent alumina, 2.5 per cent calcium oxide. and 1 per cent silica. Thepreferred compositions for the ceramic bodies of our invention, withrespect to the relative proportions of alumina, calcium oxide, andsilica, lie within this second lesser area which is wholly enclosedwithin the broader area first deilned.

In other words, the ceramic bodies of our invention contain. in general,at least about 01.5 per cent of alumina, in excess of about 0.4 per centof calcium oxide, and more than about 0.5 per cent of silica. However,the bodies preferably contain from about 92.5 to about 98.3 per cent ofalumina, from about 0.7 to about 2.5 per cent calcium oxide, and fromabout 1 to about 5 per cent ot silica. A particularly desirable bodycontains approximately 94.7 per cent alumina, 2.0 percent calcium oxide,and 3.3 per cent silica. Unless otherwise speciied, all percentagesreferred to herein will be understood to represent percentages byweight.

While the triaxlal diagram ol' Figure 1 shows the relative proportionsof alumina, calcium oxide, and silica in a. system consisting only oi'these three components. the ceramic bodies of our invention may alsoinclude small amounts of other substances. For example, we have foundthat ceramic bodies having excellent properties may be produced eventhough the altunina contains as much as 0.5 or 0.6 per cent alkali,expressed as N520. Also, we have found that the calcium oxide may beobtained from a relatively pure calcium carbonate or, if desired. from ahighcalcium limestone which may contain some incidental impurities. Fromthe diagram, however, the relative proportions that the alumina, calciumoxide, and silica in the ilnal body may bear to each other, may bedetermined regardless of what other nonessential ingredients may bepresent. In general, we prefer to keep the amounts of those nonessentialingredients as low as possible.

As a source of alumina, we may use any relatively pure grade however, weprefer to use one of the various grades manufactured by the Bayerprocess. We have found that the best results are obtained when thealumina is calclned. preferably at a temperature in excess of 2000 F..prior to use in the body. As previously discussed, the alumina used inthe ceramic bodies of our invention may contain as much as 0.5 or 0.6per cent of alkali, expressed as NaaO, without destroying the highelectrical insulating properties at elevated temperatures.

Although, as mentioned herelnbefore, the calcium oxide may be added tothe batch as calcium oxide or in any form, such as calcium carbonate orhigh-lime limestone, that will decompose durlng processing to yieldcalcium oxide, we prefer to add the calcium oxide to the body astricalcium pente-aluminate, and we have found that it is decidedlyadvantageous if the tri-calcium penta-aluminate is fused and allowed tocrystallize prior te its incorporation in the batch.

In the preparation of fused tri-calcium pentaaluminate, we prefer thatcalcium carbonate or a high-calcium quicklime be used as the source ofcalcium oxide and that Bayer process alumina be used as the source ofalumina. These materials, combined in the proper proportions, are thensintered or fused in an electric arc, or in any furnace capable ofproducing the tempera- 6 tuntoeiiectcombinamiorexamplaapmoni.- unetevm?.

Wehaveusedgiorenmplatheioliowinghateh composition for the promotion oitri-calcium pente-aluminateby iusicninadirect-areeiectric furnace:-calciuin quieklime (pebble) and 13.1 per cent Bavel' alumina(calcined). The eusedinthisbatchcontained as pereentceiciumodaandtheelupractican! impereentahmnina. cliciinnoxideandalummamay nietl einem une e man mm w Petrographic examination of some of these samplesindicates that they contain approximately 90 per eent of tricaleiumpente-aluminatc and from 5 to 10 per cent oi mono-calcium aluminate, thebalance beine essentially a mixed mass of isolated crystals and glass.

In accordance with a preferred method of practising our invention.' thetri-calcium pentaaluminate is ground so that a substantial amount willpass a B25-mesh sieve. For most purposes. we prefer that at least 90 percent oi' the material is less than B25-mesh. Either dry or wet grindingmay be employed; however, we have found that the best results can hemostreadily obtained using alcohol as a medium. The is used. is dried isused, `the same general method of preparation is followed.

Thealuminaisalsoeitherdryorwet groundso that a substantial proportion,preferably at least 99 per cent. will pass through a B25-mesh sieve.

wet-grind, using an aqueous medium: and. if this procedure is ,followedthe line alumina is dried and is then ready for use in the raw batch.

The silica to be added may be included with the alumina and ground atthe same time. or it may be ground separately, either dry or wet, sothat a substantial portion, preferably at least 99 per cent. passes a20D-mesh sieve.

The proper proportions oi alumina. tri-calcium penta-aluminate (or othermaterial forming calcium oxide on subsequent p) and silica. selected togive the desired iinal ratio as hereinbefore disclosed, are then mixedwith a suitable binder and/or plasticizer. An alternative methodistoplacetheseingredients inaballmillwith alcohol orwater and grind forashnrttimasay.

batch is added 19.3 per cent of water. of ammonium oleate. and 0.2 percent or ions mixing devices can be used; for example. both Lancaster andSimpson mixers have given good results. The mixed batch may, l! desired,thenbepassedthroug'hanaerator,dustmill.

or other suitable machine to prepare lt tor use in fabrication.

The prepared batch is then fabricated into the desired shape or into aform from which the debe obtained by a further operation, such asmachining or grinding. In some dry the fabricated piece grindingoperation. We molding to be particularly applicable. The pressure usedwill vary with the shape and the intended application of the articlebeing produced and with the amount and type of binder and piasticizerused; however, we have i'ound that pressures oi 5000 pounds per square xinch or more are applicable for most purposes.

The fabricated article may then be dried and/or burn or subjected tofurther forming operations and then dried and/or burned. The burningschedule may also vary considerably, depending upon the nal applicationfor which the ccramicbodyistobeused. Ingeneral,the schedule should besuch that uniform heating is obtained and, especially in the productionof spark-plug insulators, the time and the temperature of the burningmust be adjusted to insure the production of a non-porous product. Theporosity of the iinal body may be judged by immersion or the product inan alcohol-iuchsine dye solution at approximately 100|) pounds persquare inch pressure for about l hour. The dye penetrates the porousareas, thereby disclosing their location. We have found that thefollowing burning schedule produces excellent results coming within thescope of Heat to about 2930 F. in approximately 14 hours, hold at thattemperature for about 4 hours. and cool in place. However, burns inwhich the body was heated to 2750' F. in lVz hours, held at thattemperature for 6 hours, and led in place have produced satisfactory rc-In the practise of the present invention, we have round that thegrinding of the various components of the batch in a porcelain mill withporcelain balls tends to introduce same silica into the materials. Itwill be understood. of course. that in discussing the limits oi' thevarious constituents we refer to the total silica content of theiinished product, whether the silica is introduced separately, aspick-up during grinding, as impurities in the raw materials used, or asa combination of two or more sources. Under standard conditions. theamount oi silica introduced extraneousiy remains relativelyeonstant;therefore, the iinal silica content oi the product may be regulated bythe amount of silica added, as auch. to the raw batch.

The ed'ect oi' va ous components o! o strengths. Wit

ompressive tying the percentages of the ur ceramic body is shown in thefollowing tables. All of the bodies recorded were prepared as outlinedabove and were burned to 2930 F. in 14 h perature for 4 hours. case ofbodies navi compressive strength Table 2 shows the content on the proneaccordance with our alumina, calcium c data indicate that when nocalcium additions recorded. bonate to the bate yield calcium oxide centin the final having excellent 3.0 per cent of silica oxide content froedly alter the c up of silica during grinding, is clearly indicated; forexa 1.5 per cent of silica was ad No. B324, the iinal body c Table 3shows the eil'ect on bodies prepared from bonate, and silica. As issence of calcium oxide even oi' silica is not effective i porous body.These data bodies containing approximately 2.3 per cent calcium oxide(added as calci and cooled in piace. In the ng a.

determinations were made.

h in sumcient quantities to contents of from l to 5 per body producednonporous bodies h approximately present, varying the calcium m l to 5per cent does not markstrength. The pickdiscussed previously. mpie,although only ontained 3.0 per cent of the silica content alumina,calcium carindid, 1n the abso pente-aluminate, silica mixtures.

be noticed that bodies with reiativel are produced when the silica co asmuch as 5 per cent n producing a nonalso indicate that, in

it is added as calcium carbonate.

about 6 per cent.

TABLE 2 Eject of varying C'aO content in ceramic bodies prepared from amixture of alumina, calcium carbonate. and silica porosity may remainwith silica contents as high as 2.0 per cent. In these bodies, when thesilica content is raised to between 5 and 7.5 per cent, or higher, thestrength falls oil-rather rapidly, ours, held at that tem- 5 therebyproducing a less desirable body.

Table 4 shows the elect of calcium oxide conppreciabie porosity. no tenton bodies prepared from alumina, tri-calcium penta-aluminate, andsilica. It will be noted ei'l'ect of the calcium Oxide that, with thismixture, as little as 0.5 per cent rties oi' bodies Prepared in 10calcium oxide and 1.1 per cent silica produce invention from mixture 01'bodies having excellent properties. arbonate. and silica. These batch Na5513 gave a tra the bodies tend to be porous oxide is present.

ce ci dye penetration. wm. aiutatemi 1r-1m .if ..msi

g compress ve s reng the WMM 0f calcium caf' 16 also be noted than inthese bodies the strength tends to fall of! when the calcium oxidecontent is increased to about 3 per cent. This may be contrasted withbatch No. 5548 listed in Table 2. In general, good bodies can beproduced with somewhat higher calcium oxide contents when the calciumoxide is added as calcium carbonate than when it is added as tri-calciumpentaaiuminate. On the other hand, lower silica contents can be usedwhen the calcium oxide is ded as such to batch added as tri-calciumpenta-aluminate than when Table 5 shows the eii'ect of the silicacontent on bodies produced from alumina, tri-calcium Again it will y lowstrength ntent exceeds Tables 2, 3, 4 and 5, referred to above. appearum carbonate), some in the following pages.

Batch Composition, Burned Composition,

Per Cent Per Cent Bumd Batch No. L. C

Ahoi by maar ompre CaCO; S10; CaO S10, shrinkage, Strengt Dmefen PerCent Lb./sq. m

TABLE 3 Balch No.

Batch Com ltion, Burned Composition,

Per om Per can: Burned Linear Compreive A110: by CaC 0| Bi0| CBO S10, Se, Strength,

Dme'en Per com LbJsq. In.

1w 100 19. 1 9B 4.0 97.7 2.3 17.3 98. 5 l. 5 22. 5 98.5 3.5 95.0 5.020.9 9B. 0 4. 0 96. 2 2 3 1. 5 22. l 95. 5 4. 0 0. 5 95. 7 2. 3 2. 0 22.0 94.5 4.0 1.5 94.7 2.3 3.0 21.7 93.4 4.0 2.6 93. 7 2.3 4. 0 21.3 92.44.0 3.6 92.7 2.3 5.0 20.5 90.0 4.0 6.0 90.2 2. 3 7. 5 10. 4 87. 4 4. 08. 6 87. 7 2. 3 10. 0 19. 1

l Specially prepared alumina to give 0.0 silica content.

Tuul

ltectofoarvinacwcontentincercmicbodies from a mixture of alumina.tricalciampento-aiummte,ondaica B Y tion Batrill'ompolition,urnoilluoompod Bm am PD N" Linen mm-un um Moth! si g n.01 sommo. aio.mlm oso o. sgml-ez. Lmth me,.-- n.; as nu as 1.1 sa: man a ma...-- no anne 1.o 1.a r1.9 gaat 'im una.. aan me Y Y ou ao 1.a :1.a m o. un..- naaao ac ou 1.o n :1.a ma n. me,..- am 1an an nu an n au llamo o. ma.-- aan1an ao nu as u las man o. M 11.0v aan an 1.a as u 1x1 12am n. ma..- 1.oun an aan u u a4 reu.

Tuta

B'ectofoawingsio oontentincemmiobodies prepared from a mixture ofalumina, tricalcium pento-illuminate, and duca steh ompitim,BurnedOompclitim, B refont moms nm n Linear Comm-ive mom llolb, sfo s1.0 sommo. alo. ma oso i Pu om hummm 97.5 1.0 1.2 21.0 m Tn, 2gb: g3 agom 1.o 1.a 21.0 :naam o. 5511...-- no an 1.o oss 1.o as 22.4 man o.um,..- tao so a4 um 1.o as 21.1 ma n. 5542...-- no su afa nu 1.o 4.1:1.o man o. cao so so ne Y 1.o a1 aio mom o. 6544...-- 85.0 6.0 10.081.0 1.0 11.1 16.6 114,110 0. 1121s.,.-- aio ma n ou 2.o 1.a 21.8 annuoo. mL--- 80.0 10.0 1.0 05.7 2.8 21.4 237,0 0. mL.-. 88.0 10.0 2.0 0&72.0 8.8 21.0 238,01) 0. am lao so am zo 4.a m1 mame o. m :a et t: i: s::sa i mm2: g ma 1an su 1.a 11.2 1110 man o.

The bodies made in accordance with our invention have excellentelectrical properties. For example, a body containing approximately 95.2per cent alumina, 2 per cent calcium oxide, and 2,8 per cent silica. hada T. E. value, the temperaturc at which the material has a resistanceo1' l megohm per centimeter. in excess of 1700 F.

The thermal expansion characteristics of our body are adequate for suchapplications as sparkplug insulators. As an example, a body containingapproximately 94.7 per cent alumina, 2.0 per cent calcium oxide, and 3.3per cent silica has a mean linear coefficient o! expansion of '1.9x10per C. over the temperature range of from 0 to 950 C. The coemcient ofexpansion is relatively uniform over the range: for example, the valueis 5.6x10 over the range from 0 to 100 C.

Twenty representative samples oi our ceramic body were subjected to athermal shock test comprising rapid heating o! the tip ci an insulatorto 1601)' F., by means of an oxygen-gas torch. iollowed by rapidcooling. None of the insulators tested failed after 300 cycles of thealternate heating and cooling. This test indicates that the spellingresistance of our body is adequate for extreme service conditions. Ourceramic body is also resistant to attack by lead compounds.Repreaentative specimens showed negligible attack after immersion forone hour in molten lead oxide at 1800 F.

Petrographic examination oi nred bodies made according to our inventionindicates that they have a unique crystallization oi' the alumina.

Thebodyiscomposedoi'aslassyphaseendo! alpha alumina crystals. The amountof glassy phase varies in quantity and amount depending 'upon the amountand the ratio of calcium oxide and silica present with the alumina inthe body. It iseharacteristic oi' the products o! our invention that themass oi' alpha alumina crystals exhibit a porphyritlc structure. Theground mass is composed substantially 0i.' alpha alumina crystals ofequant form. the average diameter being ot the order oi' 10 microns orless. It is also characteristic of our invention that the remainder ofthe alpha alumina crystals are needle-iike or lath-like in form with adiameter as great as or greater than the average diameter oi thecrystals oi the ground mass and a length of at least twi their diameter.It is this structure. we believe, that imparts the superior mechanicalproperties to the body. A reproduction of a photcmicrograph ot a typicalthin-section of one of the products of our invention is shown as Figure2.

Ceramic bodies were prepared from a batch 06 containing 97.5 per centalumina and 2.5 per cent tri-calcium penta-aluminate. The burned bodiescontained approximately 08.4 per cent alumina. 0.5 per cent calciumoxide, and 1.1 per cent silica. Petrographic examination indicated thatthese 7o bodies contained about 88 per cent alpha alumina and l2 percent oi glass.

Another series of bodies was prepared from a batch composed of 88 percent oi' alumina. 10 Per cent o! trica1cium venta-aluminate, and 2 percent silica. The burned bodies contained approximately 94.7 per centalumina, 2.0 per cent calcium oxide. and 3.3 per cent silica.Petrographic examination of these bodies showed that they contained from80 to 92 per cent oi alpha alumina crystals and from 18 to 20 per centof the glass phase. The alpha alumina crystals of the ground mass wereapproximately 6 microns in diameter, while the needle-like crystals hadan average diameter oi 6 to 7 microns and an average length of 20microns. Some o! these lathlike crystals were as large as 17 by 75microns. The needle-like phase comprised from to 10 per cent o! thetotal area. No trace or gehlenite IZCaQAhOsBiCh) or anorthite(CaO.A1zOa.SiOz) was found in any of these bodies.

Another set oi bodies was prepared from a batch composed oi 40 per centalumina, 50 per cent tri-calcium pente-aluminate, and 10 per centsilica. 'Ihe final bodies contained about 89 per cent alumina. 9.5 percent calcium oxide, and 12.5 per cent silica. The burned bodies werecomposed of about 70 to 75 per cent of alpha alumina crystals and from25 to 30 per cent oi.' glass, with gehienite present in traces t0 5 percent. Part of the glass appeared to have an index of 1.638 and part hadan index of 1.60 to 1.62, revealing the presence oi' two immiscibleglasses. The coarse lath-like alpha alumina crystals were absent in thesolid matrix.

A further series oi' bodies was prepared from a batch containing 94 percent alumina, 3.4 per cent calcium carbonate. and 2.6 per cent silica.The burned bodies had an approximate composition of 94.3 per centalumina, 1.9 per cent CaO, and 3.60 per cent silica and contained about83 per cent oi alpha alumina crystals and 17 per cent glass. The alphaalumina crystals oi' the matrix averaged about 5 microns in diameter.About per cent of the area comprised the needlelike crystals having anaverage diameter greater than 15 microns. Some oi these lath-likecrystals had a diameter of 99 microns and a length ot 251 microns.

From the foregoing description o! our invention, it will be apparentthat we have provided a novel ceramic body having a novel structure. Theceramic body of our invention has high mechanical strength, goodelectrical insulating properties, good thermal conductivity, anddesirable resistance to spelling at elevated temperatures. 'Ihe body isresistant to attack at elevated temperatures by lead compounds. Aparticularly advantageous characteristic oi our body is the fact thatthe desirable properties may be obtained by burning at temperaturesbelow 3000 F.

Although we have indicated that the properties of the product of ourinvention make it particularly well suited for use as a spark-pluginsulator, it will be obvious that our ceramic body may be employed orother purposes. such, for example, as crucibles. thermo-coupleprotection tubes, extrusion dies. abrasion-resistant articles, heatingelement supports, general purpose electrical insulators, measuringgauges, nozzles and nozzle liners for use in applications involvingresistance to the erosion action oi' hot gases and liquids, etc.

While son variations have been indicated in the method and the productoi our invention, it will be readily understood that other modificationsmay be made within the scope of the appended claims.

Having thus described our invention, what we claim is:

1. A batch for the production of a ceramic body. said batch composedpredominately of alumina and containing tri-calcium penta-aluminate andsilica.

2. A batch for the production oi a ceramic body. said batch composedpredominately o! alumina and containing fused tri-calciumpenta-aluminate and silica.

3. A batch for the production of a ceramic body. said batch comprisingessentially a mixture of at least per cent of alumina, from about 2 toabout 15 per cent oi' tri-calcium penta-aluminate, and sufcient silicato give a total silica content of from about 0.5 to about 6 per centafter ilna! grinding.

4. A batch for the production of a ceramic body, said batch comprisingessentially a mixture of about B7 per cent alumina, about 10 per cent oftri-calcium penta-aluminate, and sumcient silica to give a total silicacontent oi about 3 per cent after iinal grinding.

5. A batch for the production of a ceramic body. said body comprising amaior portion of alumina and a minor portion of calcium oxide, in whichat least a part o! the calcium oxide is added to said batch astri-calcium penta-aluminate.

6. In a process for producing a ceramic body containing a maior portiono! alumina and a minor amount oi calcium oxide, the step which comprisesadding at least a part oi' the calcium oxide as trl-calciumpenta-aluminate.

'1. The method o! making a ceramic body composed essentially of alumina,lime, and silica, said method comprising the steps of preparing a iinelyground mixture of alumina, tri-calcium pentaaluminate, and silica,adding to said mixture a suitable binder and plasticizer, fabricatingthe mixture to the desired shape, and heating to a temperaturesuiiiciently high to mature the body.

8. A batch for the production of a burned ceramic body. said batchcomposed predominantly oi' alumina and containing tri-calciumpento-aluminate and silica in amounts sumcient to give a burned body inwhich the relative percentages of said three ingredients lie within thearea dened approximately in the accompanying Figure 1 by the solid linesAB, BC, CD, DE and EA.

9. A batch for the production of a burned ceramic body, said batchcomposed predominantly of alumina and containing fused tri-calciumpenta-aluminate and silica in amounts suiiicient to give a burned bodyin which the relative percentages of said three ingredients lie withinthe area deilned approximately in the accompanying Figure 1 by thebroken lines FG, GH, HI and Il".

(2mm R. AUSTIN. EDWIN J. ROGERS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATFS PATENTS Number Name Date 954,808 Jeppson et al, Apr. 12,1910 1,528,639 Tone Mar. 3. 1925 1,741,920 Curtis Dec. 31, 1929 954,766Saunders Apr. 12, 1910 1,931,795 Heilman Oct. 24, 1933 1,942,431 JungJan. 9, 1934 1,966,497 Hauman July 10, 1934 1,966,408 Hauman July 10,1934 2,019,299 Benner et al. Oct. 29, 1935 (Other references onfollowing page) 13 14 Number Name Date Number Name Date 71,229 SeymourNov. 19, 1867 2,277,705 Kmzle et al Mar. 31, 1942 1,942,879 Riddle Jan.9, 1934 2,341,561 Kinzie et al. Feb. 15, 1944 2,043,029 Blau et al. June2, 1936 2,277,735 Wamel' et al. MM'. 31, 1942 2,047,457 Benner et a!July 14, 1936 2,323,951 Wamel July 13, 1043 269,236 Klingler et al May1, 2,207,558 Sulger July 9, 1940 2,272,018 F'essler et al Feb. 0,2,331,232 Ross oct. 5, 1943 FOREIGN PATENTS 2,220,411 Kinzle et al. Nov.5, 1040 Number Country Date 2,220,412 Klnzie et al Nov. 5, 1940 10373.111 Italy 1939 Certificate of Correction Patent No. 2,423,958. July15, 1947. CHESTER R. AUSTIN ET AL.

It is hereby certified that errors appear in the printed specificationof the above numbered patent requiring correction as follows: Column 3,line 14, for "clamed reed claimed; line 21, for desire read desired;column 6, line 63, for same read some; column 10, line 42, for end readand; and that the said Letters Patent should be read with thesecorrections therein that the same may conform to the record of the casein the Patent Office.

Signed and sealed this 30th day of September, A. D. 1947.

THOMAS F. MURPHY,

Assistant ommzuioner of Patents.

13 14 Number Name Date Number Name Date 71,229 Seymour Nov. 19, 18672,277,705 Kmzle et al Mar. 31, 1942 1,942,879 Riddle Jan. 9, 19342,341,561 Kinzie et al. Feb. 15, 1944 2,043,029 Blau et al. June 2, 19362,277,735 Wamel' et al. MM'. 31, 1942 2,047,457 Benner et a! July 14,1936 2,323,951 Wamel July 13, 1043 269,236 Klingler et al May 1,2,207,558 Sulger July 9, 1940 2,272,018 F'essler et al Feb. 0, 2,331,232Ross oct. 5, 1943 FOREIGN PATENTS 2,220,411 Kinzle et al. Nov. 5, 1040Number Country Date 2,220,412 Klnzie et al Nov. 5, 1940 10 373.111 Italy1939 Certificate of Correction Patent No. 2,423,958. July 15, 1947.CHESTER R. AUSTIN ET AL.

It is hereby certified that errors appear in the printed specificationof the above numbered patent requiring correction as follows: Column 3,line 14, for "clamed reed claimed; line 21, for desire read desired;column 6, line 63, for same read some; column 10, line 42, for end readand; and that the said Letters Patent should be read with thesecorrections therein that the same may conform to the record of the casein the Patent Office.

Signed and sealed this 30th day of September, A. D. 1947.

THOMAS F. MURPHY,

Assistant ommzuioner of Patents.

